Utilizing cached encoded data slices in a dispersed storage network

ABSTRACT

A method begins with a processing module of a dispersed storage network (DSN) executing a write operation to write a plurality of sets of encoded data slices to DSN memory, where the write operation includes temporarily storing encoded data slices in cache memory. The processing module receives a read request for a portion of the data object and determines whether a decode threshold number of encoded data slices of a set is stored in the cache memory. When less than the decode threshold number of encoded data slices is stored in the cache memory, the processing module retrieves encoded data slices from the DSN memory to obtain the decode threshold number of encoded data slices, decodes the decode threshold number of encoded data slices to recover a corresponding data segment, and outputs the recovered corresponding data segment.

CROSS REFERENCE TO RELATED PATENTS

The present U.S. Utility patent application claims priority pursuant to35 U.S.C. §120 as a continuation-in-part of U.S. Utility applicationSer. No. 14/148,652, entitled “ADJUSTING DATA DISPERSAL IN A DISPERSEDSTORAGE NETWORK,” filed Jan. 6, 2014, pending, which is a continuationof U.S. Utility application Ser. No. 13/946,136, entitled “ADJUSTINGDATA DISPERSAL IN A DISPERSED STORAGE NETWORK,” filed Jul. 19, 2013, nowU.S. Pat. No. 8,627,178, issued on Jan. 7, 2014, which is a continuationof U.S. Utility application Ser. No. 12/983,214, entitled “ADJUSTINGDATA DISPERSAL IN A DISPERSED STORAGE NETWORK,” filed Dec. 31, 2010, nowU.S. Pat. No. 8,495,466, issued on Jul. 23, 2013, which claims prioritypursuant to 35 U.S.C. §119(e) to U.S. Provisional Application No.61/314,166, entitled “STORAGE AND RETRIEVAL IN A DISTRIBUTED STORAGESYSTEM,” filed Mar. 16, 2010, all of which are hereby incorporatedherein by reference in their entirety and made part of the present U.S.Utility patent application for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT—NOTAPPLICABLE INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACTDISC—NOT APPLICABLE BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

This invention relates generally to computing systems and moreparticularly to data storage solutions within such computing systems.

2. Description of Related Art

Computers are known to communicate, process, and store data. Suchcomputers range from wireless smart phones to data centers that supportmillions of web searches, stock trades, or on-line purchases every day.In general, a computing system generates data and/or manipulates datafrom one form into another. For instance, an image sensor of thecomputing system generates raw picture data and, using an imagecompression program (e.g., JPEG, MPEG, etc.), the computing systemmanipulates the raw picture data into a standardized compressed image.

With continued advances in processing speed and communication speed,computers are capable of processing real time multimedia data forapplications ranging from simple voice communications to streaming highdefinition video. As such, general-purpose information appliances arereplacing purpose-built communications devices (e.g., a telephone). Forexample, smart phones can support telephony communications but they arealso capable of text messaging and accessing the internet to performfunctions including email, web browsing, remote applications access, andmedia communications (e.g., telephony voice, image transfer, musicfiles, video files, real time video streaming. etc.).

Each type of computer is constructed and operates in accordance with oneor more communication, processing, and storage standards. As a result ofstandardization and with advances in technology, more and moreinformation content is being converted into digital formats. Forexample, more digital cameras are now being sold than film cameras, thusproducing more digital pictures. As another example, web-basedprogramming is becoming an alternative to over the air televisionbroadcasts and/or cable broadcasts. As further examples, papers, books,video entertainment, home video, etc. are now being stored digitally,which increases the demand on the storage function of computers.

A typical computer storage system includes one or more memory devicesaligned with the needs of the various operational aspects of thecomputer's processing and communication functions. Generally, theimmediacy of access dictates what type of memory device is used. Forexample, random access memory (RAM) memory can be accessed in any randomorder with a constant response time, thus it is typically used for cachememory and main memory. By contrast, memory device technologies thatrequire physical movement such as magnetic disks, tapes, and opticaldiscs, have a variable response time as the physical movement can takelonger than the data transfer, thus they are typically used forsecondary memory (e.g., hard drive, backup memory, etc.).

A computer's storage system will be compliant with one or more computerstorage standards that include, but are not limited to, network filesystem (NFS), flash file system (FFS), disk file system (DFS), smallcomputer system interface (SCSI), internet small computer systeminterface (iSCSI), file transfer protocol (FTP), and web-baseddistributed authoring and versioning (WebDAV). These standards specifythe data storage format (e.g., files, data objects, data blocks,directories, etc.) and interfacing between the computer's processingfunction and its storage system, which is a primary function of thecomputer's memory controller.

Despite the standardization of the computer and its storage system,memory devices fail; especially commercial grade memory devices thatutilize technologies incorporating physical movement (e.g., a discdrive). For example, it is fairly common for a disc drive to routinelysuffer from bit level corruption and to completely fail after threeyears of use. One solution is to a higher-grade disc drive, which addssignificant cost to a computer.

Another solution is to utilize multiple levels of redundant disc drivesto replicate the data into two or more copies. One such redundant driveapproach is called redundant array of independent discs (RAID). In aRAID device, a RAID controller adds parity data to the original databefore storing it across the array. The parity data is calculated fromthe original data such that the failure of a disc will not result in theloss of the original data. For example, RAID 5 uses three discs toprotect data from the failure of a single disc. The parity data, andassociated redundancy overhead data, reduces the storage capacity ofthree independent discs by one third (e.g., n−1=capacity). RAID 6 canrecover from a loss of two discs and requires a minimum of four discswith a storage capacity of n−2.

While RAID addresses the memory device failure issue, it is not withoutits own failures issues that affect its effectiveness, efficiency andsecurity. For instance, as more discs are added to the array, theprobability of a disc failure increases, which increases the demand formaintenance. For example, when a disc fails, it needs to be manuallyreplaced before another disc fails and the data stored in the RAIDdevice is lost. To reduce the risk of data loss, data on a RAID deviceis typically copied on to one or more other RAID devices. While thisaddresses the loss of data issue, it raises a security issue sincemultiple copies of data are available, which increases the chances ofunauthorized access. Further, as the amount of data being stored grows,the overhead of RAID devices becomes a non-trivial efficiency issue.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 is a schematic block diagram of an embodiment of a computingsystem in accordance with the present invention;

FIG. 2 is a schematic block diagram of an embodiment of a computing corein accordance with the present invention;

FIG. 3 is a schematic block diagram of an embodiment of a distributedstorage processing unit in accordance with the present invention;

FIG. 4 is a schematic block diagram of an embodiment of a grid module inaccordance with the present invention;

FIG. 5 is a diagram of an example embodiment of error coded data slicecreation in accordance with the present invention;

FIG. 6 is a flowchart illustrating an example of storing data as encodeddata slices in accordance with present invention;

FIG. 7 is a flowchart illustrating an example of re-storing data asencoded data slices in accordance with present invention;

FIG. 8A is a diagram illustrating an example of data segmentation ofdata in accordance with the present invention;

FIG. 8B is a diagram illustrating an example of encoding data segmentsin accordance with the present invention;

FIG. 9 is a flowchart illustrating an example of segmenting data inaccordance with the present invention;

FIG. 10 is a flowchart illustrating an example of re-creating data inaccordance with the present invention;

FIG. 11 is a flowchart illustrating an example of caching encoded dataslices in accordance with the present invention;

FIGS. 12A-C are schematic block diagrams of an embodiment of a dispersedstorage network (DSN) in accordance with the present invention;

FIG. 12D is a flowchart illustrating an example of caching rebuiltencoded data slices in accordance with the present invention;

FIG. 12E is a flowchart illustrating an example of caching rebuiltencoded data slices in accordance with the present invention;

FIGS. 13A-C are schematic block diagrams of another embodiment of adispersed storage network (DSN) in accordance with the presentinvention;

FIG. 13D is a flowchart illustrating another example of caching encodeddata slices in accordance with the present invention; and

FIG. 13E is a flowchart illustrating another example of caching encodeddata slices in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic block diagram of a computing system 10 thatincludes one or more of a first type of user devices 12, one or more ofa second type of user devices 14, at least one distributed storage (DS)processing unit 16, at least one DS managing unit 18, at least onestorage integrity processing unit 20, and a distributed storage network(DSN) memory 22 coupled via a network 24. The network 24 may include oneor more wireless and/or wire lined communication systems; one or moreprivate intranet systems and/or public internet systems; and/or one ormore local area networks (LAN) and/or wide area networks (WAN).

The DSN memory 22 includes a plurality of distributed storage (DS) units36 for storing data of the system. Each of the DS units 36 includes aprocessing module and memory and may be located at a geographicallydifferent site than the other DS units (e.g., one in Chicago, one inMilwaukee, etc.). The processing module may be a single processingdevice or a plurality of processing devices. Such a processing devicemay be a microprocessor, micro-controller, digital signal processor,microcomputer, central processing unit, field programmable gate array,programmable logic device, state machine, logic circuitry, analogcircuitry, digital circuitry, and/or any device that manipulates signals(analog and/or digital) based on hard coding of the circuitry and/oroperational instructions. The processing module may have an associatedmemory and/or memory element, which may be a single memory device, aplurality of memory devices, and/or embedded circuitry of the processingmodule. Such a memory device may be a read-only memory, random accessmemory, volatile memory, non-volatile memory, static memory, dynamicmemory, flash memory, cache memory, and/or any device that storesdigital information. Note that if the processing module includes morethan one processing device, the processing devices may be centrallylocated (e.g., directly coupled together via a wired and/or wireless busstructure) or may be distributedly located (e.g., cloud computing viaindirect coupling via a local area network and/or a wide area network).Further note that when the processing module implements one or more ofits functions via a state machine, analog circuitry, digital circuitry,and/or logic circuitry, the memory and/or memory element storing thecorresponding operational instructions may be embedded within, orexternal to, the circuitry comprising the state machine, analogcircuitry, digital circuitry, and/or logic circuitry. Still further notethat, the memory element stores, and the processing module executes,hard coded and/or operational instructions corresponding to at leastsome of the steps and/or functions illustrated in FIGS. 1-13.

Each of the user devices 12-14, the DS processing unit 16, the DSmanaging unit 18, and the storage integrity processing unit 20 may be aportable computing device (e.g., a social networking device, a gamingdevice, a cell phone, a smart phone, a personal digital assistant, adigital music player, a digital video player, a laptop computer, ahandheld computer, a video game controller, and/or any other portabledevice that includes a computing core) and/or a fixed computing device(e.g., a personal computer, a computer server, a cable set-top box, asatellite receiver, a television set, a printer, a fax machine, homeentertainment equipment, a video game console, and/or any type of homeor office computing equipment). Such a portable or fixed computingdevice includes a computing core 26 and one or more interfaces 30, 32,and/or 33. An embodiment of the computing core 26 will be described withreference to FIG. 2.

With respect to the interfaces, each of the interfaces 30, 32, and 33includes software and/or hardware to support one or more communicationlinks via the network 24 and/or directly. For example, interfaces 30support a communication link (wired, wireless, direct, via a LAN, viathe network 24, etc.) between the first type of user device 12 and theDS processing unit 16. As another example, DSN interface 32 supports aplurality of communication links via the network 24 between the DSNmemory 22 and the DS processing unit 16, the first type of user device12, and/or the storage integrity processing unit 20. As yet anotherexample, interface 33 supports a communication link between the DSmanaging unit 18 and any one of the other devices and/or units 12, 14,16, 20, and/or 22 via the network 24.

In general and with respect to data storage, the system 10 supportsthree primary functions: distributed network data storage management,distributed data storage and retrieval, and data storage integrityverification. In accordance with these three primary functions, data canbe distributedly stored in a plurality of physically different locationsand subsequently retrieved in a reliable and secure manner regardless offailures of individual storage devices, failures of network equipment,the duration of storage, the amount of data being stored, attempts athacking the data, etc.

The DS managing unit 18 performs distributed network data storagemanagement functions, which include establishing distributed datastorage parameters, performing network operations, performing networkadministration, and/or performing network maintenance. The DS managingunit 18 establishes the distributed data storage parameters (e.g.,allocation of virtual DSN memory space, distributed storage parameters,security parameters, billing information, user profile information,etc.) for one or more of the user devices 12-14 (e.g., established forindividual devices, established for a user group of devices, establishedfor public access by the user devices, etc.). For example, the DSmanaging unit 18 coordinates the creation of a vault (e.g., a virtualmemory block) within the DSN memory 22 for a user device (for a group ofdevices, or for public access). The DS managing unit 18 also determinesthe distributed data storage parameters for the vault. In particular,the DS managing unit 18 determines a number of slices (e.g., the numberthat a data segment of a data file and/or data block is partitioned intofor distributed storage) and a read threshold value (e.g., the minimumnumber of slices required to reconstruct the data segment).

As another example, the DS managing module 18 creates and stores,locally or within the DSN memory 22, user profile information. The userprofile information includes one or more of authentication information,permissions, and/or the security parameters. The security parameters mayinclude one or more of encryption/decryption scheme, one or moreencryption keys, key generation scheme, and data encoding/decodingscheme.

As yet another example, the DS managing unit 18 creates billinginformation for a particular user, user group, vault access, publicvault access, etc. For instance, the DS managing unit 18 tracks thenumber of times user accesses a private vault and/or public vaults,which can be used to generate a per-access bill. In another instance,the DS managing unit 18 tracks the amount of data stored and/orretrieved by a user device and/or a user group, which can be used togenerate a per-data-amount bill.

The DS managing unit 18 also performs network operations, networkadministration, and/or network maintenance. As at least part ofperforming the network operations and/or administration, the DS managingunit 18 monitors performance of the devices and/or units of the system10 for potential failures, determines the devices and/or unit'sactivation status, determines the devices' and/or units' loading, andany other system level operation that affects the performance level ofthe system 10. For example, the DS managing unit 18 receives andaggregates network management alarms, alerts, errors, statusinformation, performance information, and messages from the devices12-14 and/or the units 16, 20, 22. For example, the DS managing unit 18receives a simple network management protocol (SNMP) message regardingthe status of the DS processing unit 16.

The DS managing unit 18 performs the network maintenance by identifyingequipment within the system 10 that needs replacing, upgrading,repairing, and/or expanding. For example, the DS managing unit 18determines that the DSN memory 22 needs more DS units 36 or that one ormore of the DS units 36 needs updating.

The second primary function (i.e., distributed data storage andretrieval) begins and ends with a user device 12-14. For instance, if asecond type of user device 14 has a data file 38 and/or data block 40 tostore in the DSN memory 22, it send the data file 38 and/or data block40 to the DS processing unit 16 via its interface 30. As will bedescribed in greater detail with reference to FIG. 2, the interface 30functions to mimic a conventional operating system (OS) file systeminterface (e.g., network file system (NFS), flash file system (FFS),disk file system (DFS), file transfer protocol (FTP), web-baseddistributed authoring and versioning (WebDAV), etc.) and/or a blockmemory interface (e.g., small computer system interface (SCSI), internetsmall computer system interface (iSCSI), etc.). In addition, theinterface 30 may attach a user identification code (ID) to the data file38 and/or data block 40.

The DS processing unit 16 receives the data file 38 and/or data block 40via its interface 30 and performs a distributed storage (DS) process 34thereon (e.g., an error coding dispersal storage function). The DSprocessing 34 begins by partitioning the data file 38 and/or data block40 into one or more data segments, which is represented as Y datasegments. For example, the DS processing 34 may partition the data file38 and/or data block 40 into a fixed byte size segment (e.g., 2¹ to2^(n) bytes, where n=>2) or a variable byte size (e.g., change byte sizefrom segment to segment, or from groups of segments to groups ofsegments, etc.).

For each of the Y data segments, the DS processing 34 error encodes(e.g., forward error correction (FEC), information dispersal algorithm,or error correction coding) and slices (or slices then error encodes)the data segment into a plurality of error coded (EC) data slices 42-48,which is represented as X slices per data segment. The number of slices(X) per segment, which corresponds to a number of pillars n, is set inaccordance with the distributed data storage parameters and the errorcoding scheme. For example, if a Reed-Solomon (or other FEC scheme) isused in an n/k system, then a data segment is divided into n slices,where k number of slices is needed to reconstruct the original data(i.e., k is the threshold). As a few specific examples, the n/k factormay be 5/3; 6/4; 8/6; 8/5; 16/10.

For each slice 42-48, the DS processing unit 16 creates a unique slicename and appends it to the corresponding slice 42-48. The slice nameincludes universal DSN memory addressing routing information (e.g.,virtual memory addresses in the DSN memory 22) and user-specificinformation (e.g., user ID, file name, data block identifier, etc.).

The DS processing unit 16 transmits the plurality of EC slices 42-48 toa plurality of DS units 36 of the DSN memory 22 via the DSN interface 32and the network 24. The DSN interface 32 formats each of the slices fortransmission via the network 24. For example, the DSN interface 32 mayutilize an internet protocol (e.g., TCP/IP, etc.) to packetize theslices 42-48 for transmission via the network 24.

The number of DS units 36 receiving the slices 42-48 is dependent on thedistributed data storage parameters established by the DS managing unit18. For example, the DS managing unit 18 may indicate that each slice isto be stored in a different DS unit 36. As another example, the DSmanaging unit 18 may indicate that like slice numbers of different datasegments are to be stored in the same DS unit 36. For example, the firstslice of each of the data segments is to be stored in a first DS unit36, the second slice of each of the data segments is to be stored in asecond DS unit 36, etc. In this manner, the data is encoded anddistributedly stored at physically diverse locations to improved datastorage integrity and security. Further examples of encoding the datasegments will be provided with reference to one or more of FIGS. 2-13.

Each DS unit 36 that receives a slice 42-48 for storage translates thevirtual DSN memory address of the slice into a local physical addressfor storage. Accordingly, each DS unit 36 maintains a virtual tophysical memory mapping to assist in the storage and retrieval of data.

The first type of user device 12 performs a similar function to storedata in the DSN memory 22 with the exception that it includes the DSprocessing. As such, the device 12 encodes and slices the data fileand/or data block it has to store. The device then transmits the slices11 to the DSN memory via its DSN interface 32 and the network 24.

For a second type of user device 14 to retrieve a data file or datablock from memory, it issues a read command via its interface 30 to theDS processing unit 16. The DS processing unit 16 performs the DSprocessing 34 to identify the DS units 36 storing the slices of the datafile and/or data block based on the read command. The DS processing unit16 may also communicate with the DS managing unit 18 to verify that theuser device 14 is authorized to access the requested data.

Assuming that the user device is authorized to access the requesteddata, the DS processing unit 16 issues slice read commands to at least athreshold number of the DS units 36 storing the requested data (e.g., toat least 10 DS units for a 16/10 error coding scheme). Each of the DSunits 36 receiving the slice read command, verifies the command,accesses its virtual to physical memory mapping, retrieves the requestedslice, or slices, and transmits it to the DS processing unit 16.

Once the DS processing unit 16 has received a read threshold number ofslices for a data segment, it performs an error decoding function andde-slicing to reconstruct the data segment. When Y number of datasegments has been reconstructed, the DS processing unit 16 provides thedata file 38 and/or data block 40 to the user device 14. Note that thefirst type of user device 12 performs a similar process to retrieve adata file and/or data block.

The storage integrity processing unit 20 performs the third primaryfunction of data storage integrity verification. In general, the storageintegrity processing unit 20 periodically retrieves slices 45, and/orslice names, of a data file or data block of a user device to verifythat one or more slices have not been corrupted or lost (e.g., the DSunit failed). The retrieval process mimics the read process previouslydescribed.

If the storage integrity processing unit 20 determines that one or moreslices is corrupted or lost, it rebuilds the corrupted or lost slice(s)in accordance with the error coding scheme. The storage integrityprocessing unit 20 stores the rebuild slice, or slices, in theappropriate DS unit(s) 36 in a manner that mimics the write processpreviously described.

FIG. 2 is a schematic block diagram of an embodiment of a computing core26 that includes a processing module 50, a memory controller 52, mainmemory 54, a video graphics processing unit 55, an input/output (TO)controller 56, a peripheral component interconnect (PCI) interface 58,at least one IO device interface module 62, a read only memory (ROM)basic input output system (BIOS) 64, and one or more memory interfacemodules. The memory interface module(s) includes one or more of auniversal serial bus (USB) interface module 66, a host bus adapter (HBA)interface module 68, a network interface module 70, a flash interfacemodule 72, a hard drive interface module 74, and a DSN interface module76. Note the DSN interface module 76 and/or the network interface module70 may function as the interface 30 of the user device 14 of FIG. 1.Further note that the IO device interface module 62 and/or the memoryinterface modules may be collectively or individually referred to as IOports.

The processing module 50 may be a single processing device or aplurality of processing devices. Such a processing device may be amicroprocessor, micro-controller, digital signal processor,microcomputer, central processing unit, field programmable gate array,programmable logic device, state machine, logic circuitry, analogcircuitry, digital circuitry, and/or any device that manipulates signals(analog and/or digital) based on hard coding of the circuitry and/oroperational instructions. The processing module 50 may have anassociated memory and/or memory element, which may be a single memorydevice, a plurality of memory devices, and/or embedded circuitry of theprocessing module 50. Such a memory device may be a read-only memory,random access memory, volatile memory, non-volatile memory, staticmemory, dynamic memory, flash memory, cache memory, and/or any devicethat stores digital information. Note that if the processing module 50includes more than one processing device, the processing devices may becentrally located (e.g., directly coupled together via a wired and/orwireless bus structure) or may be distributedly located (e.g., cloudcomputing via indirect coupling via a local area network and/or a widearea network). Further note that when the processing module 50implements one or more of its functions via a state machine, analogcircuitry, digital circuitry, and/or logic circuitry, the memory and/ormemory element storing the corresponding operational instructions may beembedded within, or external to, the circuitry comprising the statemachine, analog circuitry, digital circuitry, and/or logic circuitry.Still further note that, the memory element stores, and the processingmodule 50 executes, hard coded and/or operational instructionscorresponding to at least some of the steps and/or functions illustratedin FIGS. 1-13.

FIG. 3 is a schematic block diagram of an embodiment of a dispersedstorage (DS) processing module 34 of user device 12 and/or of the DSprocessing unit 16. The DS processing module 34 includes a gatewaymodule 78, an access module 80, a grid module 82, and a storage module84. The DS processing module 34 may also include an interface 30 and theDSnet interface 32 or the interfaces 68 and/or 70 may be part of user 12or of the DS processing unit 14. The DS processing module 34 may furtherinclude a bypass/feedback path between the storage module 84 to thegateway module 78. Note that the modules 78-84 of the DS processingmodule 34 may be in a single unit or distributed across multiple units.

In an example of storing data, the gateway module 78 receives anincoming data object that includes a user ID field 86, an object namefield 88, and the data field 40 and may also receive correspondinginformation that includes a process identifier (e.g., an internalprocess/application ID), metadata, a file system directory, a blocknumber, a transaction message, a user device identity (ID), a dataobject identifier, a source name, and/or user information. The gatewaymodule 78 authenticates the user associated with the data object byverifying the user ID 86 with the managing unit 18 and/or anotherauthenticating unit.

When the user is authenticated, the gateway module 78 obtains userinformation from the management unit 18, the user device, and/or theother authenticating unit. The user information includes a vaultidentifier, operational parameters, and user attributes (e.g., userdata, billing information, etc.). A vault identifier identifies a vault,which is a virtual memory space that maps to a set of DS storage units36. For example, vault 1 (i.e., user 1's DSN memory space) includeseight DS storage units (X=8 wide) and vault 2 (i.e., user 2's DSN memoryspace) includes sixteen DS storage units (X=16 wide). The operationalparameters may include an error coding algorithm, the width n (number ofpillars X or slices per segment for this vault), a read threshold T, awrite threshold, an encryption algorithm, a slicing parameter, acompression algorithm, an integrity check method, caching settings,parallelism settings, and/or other parameters that may be used to accessthe DSN memory layer.

The gateway module 78 uses the user information to assign a source name35 to the data. For instance, the gateway module 60 determines thesource name 35 of the data object 40 based on the vault identifier andthe data object. For example, the source name may contain a fileidentifier (ID), a vault generation number, a reserved field, and avault identifier (ID). As another example, the gateway module 78 maygenerate the file ID based on a hash function of the data object 40.Note that the gateway module 78 may also perform message conversion,protocol conversion, electrical conversion, optical conversion, accesscontrol, user identification, user information retrieval, trafficmonitoring, statistics generation, configuration, management, and/orsource name determination.

The access module 80 receives the data object 40 and creates a series ofdata segments 1 through Y 90-92 in accordance with a data storageprotocol (e.g., file storage system, a block storage system, and/or anaggregated block storage system). The number of segments Y may be chosenor randomly assigned based on a selected segment size and the size ofthe data object. For example, if the number of segments is chosen to bea fixed number, then the size of the segments varies as a function ofthe size of the data object. For instance, if the data object is animage file of 4,194,304 eight bit bytes (e.g., 33,554,432 bits) and thenumber of segments Y=131,072, then each segment is 256 bits or 32 bytes.As another example, if segment sized is fixed, then the number ofsegments Y varies based on the size of data object. For instance, if thedata object is an image file of 4,194,304 bytes and the fixed size ofeach segment is 4,096 bytes, the then number of segments Y=1,024. Notethat each segment is associated with the same source name.

The grid module 82 receives the data segments and may manipulate (e.g.,compression, encryption, cyclic redundancy check (CRC), etc.) each ofthe data segments before performing an error coding function of theerror coding dispersal storage function to produce a pre-manipulateddata segment. After manipulating a data segment, if applicable, the gridmodule 82 error encodes (e.g., Reed-Solomon, Convolution encoding,Trellis encoding, etc.) the data segment or manipulated data segmentinto X error coded data slices 42-44.

The value X, or the number of pillars (e.g., X=16), is chosen as aparameter of the error coding dispersal storage function. Otherparameters of the error coding dispersal function include a readthreshold T, a write threshold W, etc. The read threshold (e.g., T=10,when X=16) corresponds to the minimum number of error-free error codeddata slices required to reconstruct the data segment. In other words,the DS processing module 34 can compensate for X−T (e.g., 16−10=6)missing error coded data slices per data segment. The write threshold Wcorresponds to a minimum number of DS storage units that acknowledgeproper storage of their respective data slices before the DS processingmodule indicates proper storage of the encoded data segment. Note thatthe write threshold is greater than or equal to the read threshold for agiven number of pillars (X).

For each data slice of a data segment, the grid module 82 generates aunique slice name 37 and attaches it thereto. The slice name 37 includesa universal routing information field and a vault specific field and maybe 48 bytes (e.g., 24 bytes for each of the universal routinginformation field and the vault specific field). As illustrated, theuniversal routing information field includes a slice index, a vault ID,a vault generation, and a reserved field. The slice index is based onthe pillar number and the vault ID and, as such, is unique for eachpillar (e.g., slices of the same pillar for the same vault for anysegment will share the same slice index). The vault specific fieldincludes a data name, which includes a file ID and a segment number(e.g., a sequential numbering of data segments 1-Y of a simple dataobject or a data block number).

Prior to outputting the error coded data slices of a data segment, thegrid module may perform post-slice manipulation on the slices. Ifenabled, the manipulation includes slice level compression, encryption,CRC, addressing, tagging, and/or other manipulation to improve theeffectiveness of the computing system.

When the error coded data slices of a data segment are ready to beoutputted, the grid module 82 determines which of the DS storage units36 will store the EC data slices based on a dispersed storage memorymapping associated with the user's vault and/or DS storage unitattributes. The DS storage unit attributes may include availability,self-selection, performance history, link speed, link latency,ownership, available DSN memory, domain, cost, a prioritization scheme,a centralized selection message from another source, a lookup table,data ownership, and/or any other factor to optimize the operation of thecomputing system. Note that the number of DS storage units 36 is equalto or greater than the number of pillars (e.g., X) so that no more thanone error coded data slice of the same data segment is stored on thesame DS storage unit 36. Further note that EC data slices of the samepillar number but of different segments (e.g., EC data slice 1 of datasegment 1 and EC data slice 1 of data segment 2) may be stored on thesame or different DS storage units 36.

The storage module 84 performs an integrity check on the outboundencoded data slices and, when successful, identifies a plurality of DSstorage units based on information provided by the grid module 82. Thestorage module 84 then outputs the encoded data slices 1 through X ofeach segment 1 through Y to the DS storage units 36. Each of the DSstorage units 36 stores its EC data slice(s) and maintains a localvirtual DSN address to physical location table to convert the virtualDSN address of the EC data slice(s) into physical storage addresses.

In an example of a read operation, the user device 12 and/or 14 sends aread request to the DS processing unit 14, which authenticates therequest. When the request is authentic, the DS processing unit 14 sendsa read message to each of the DS storage units 36 storing slices of thedata object being read. The slices are received via the DSnet interface32 and processed by the storage module 84, which performs a parity checkand provides the slices to the grid module 82 when the parity check wassuccessful. The grid module 82 decodes the slices in accordance with theerror coding dispersal storage function to reconstruct the data segment.The access module 80 reconstructs the data object from the data segmentsand the gateway module 78 formats the data object for transmission tothe user device.

FIG. 4 is a schematic block diagram of an embodiment of a grid module 82that includes a control unit 73, a pre-slice manipulator 75, an encoder77, a slicer 79, a post-slice manipulator 81, a pre-slice de-manipulator83, a decoder 85, a de-slicer 87, and/or a post-slice de-manipulator 89.Note that the control unit 73 may be partially or completely external tothe grid module 82. For example, the control unit 73 may be part of thecomputing core at a remote location, part of a user device, part of theDS managing unit 18, or distributed amongst one or more DS storageunits.

In an example of write operation, the pre-slice manipulator 75 receivesa data segment 90-92 and a write instruction from an authorized userdevice. The pre-slice manipulator 75 determines if pre-manipulation ofthe data segment 90-92 is required and, if so, what type. The pre-slicemanipulator 75 may make the determination independently or based oninstructions from the control unit 73, where the determination is basedon a computing system-wide predetermination, a table lookup, vaultparameters associated with the user identification, the type of data,security requirements, available DSN memory, performance requirements,and/or other metadata.

Once a positive determination is made, the pre-slice manipulator 75manipulates the data segment 90-92 in accordance with the type ofmanipulation. For example, the type of manipulation may be compression(e.g., Lempel-Ziv-Welch, Huffman, Golomb, fractal, wavelet, etc.),signatures (e.g., Digital Signature Algorithm (DSA), Elliptic Curve DSA,Secure Hash Algorithm, etc.), watermarking, tagging, encryption (e.g.,Data Encryption Standard, Advanced Encryption Standard, etc.), addingmetadata (e.g., time/date stamping, user information, file type, etc.),cyclic redundancy check (e.g., CRC32), and/or other data manipulationsto produce the pre-manipulated data segment.

The encoder 77 encodes the pre-manipulated data segment 92 using aforward error correction (FEC) encoder (and/or other type of erasurecoding and/or error coding) to produce an encoded data segment 94. Theencoder 77 determines which forward error correction algorithm to usebased on a predetermination associated with the user's vault, a timebased algorithm, user direction, DS managing unit direction, controlunit direction, as a function of the data type, as a function of thedata segment 92 metadata, and/or any other factor to determine algorithmtype. The forward error correction algorithm may be Golay,Multidimensional parity, Reed-Solomon, Hamming, Bose Ray ChauduriHocquenghem (BCH), Cauchy-Reed-Solomon, or any other FEC encoder. Notethat the encoder 77 may use a different encoding algorithm for each datasegment 92, the same encoding algorithm for the data segments 92 of adata object, or a combination thereof.

The encoded data segment 94 is of greater size than the data segment 92by the overhead rate of the encoding algorithm by a factor of X/T, whereX is the width or number of slices, and T is the read threshold. In thisregard, the corresponding decoding process can accommodate at most X−Tmissing EC data slices and still recreate the data segment 92. Forexample, if X=16 and T=10, then the data segment 92 will be recoverableas long as 10 or more EC data slices per segment are not corrupted.

The slicer 79 transforms the encoded data segment 94 into EC data slicesin accordance with the slicing parameter from the vault for this userand/or data segment 92. For example, if the slicing parameter is X=16,then the slicer 79 slices each encoded data segment 94 into 16 encodedslices.

The post-slice manipulator 81 performs, if enabled, post-manipulation onthe encoded slices to produce the EC data slices. If enabled, thepost-slice manipulator 81 determines the type of post-manipulation,which may be based on a computing system-wide predetermination,parameters in the vault for this user, a table lookup, the useridentification, the type of data, security requirements, available DSNmemory, performance requirements, control unit directed, and/or othermetadata. Note that the type of post-slice manipulation may includeslice level compression, signatures, encryption, CRC, addressing,watermarking, tagging, adding metadata, and/or other manipulation toimprove the effectiveness of the computing system.

In an example of a read operation, the post-slice de-manipulator 89receives at least a read threshold number of EC data slices and performsthe inverse function of the post-slice manipulator 81 to produce aplurality of encoded slices. The de-slicer 87 de-slices the encodedslices to produce an encoded data segment 94. The decoder 85 performsthe inverse function of the encoder 77 to recapture the data segment90-92. The pre-slice de-manipulator 83 performs the inverse function ofthe pre-slice manipulator 75 to recapture the data segment 90-92.

FIG. 5 is a diagram of an example of slicing an encoded data segment 94by the slicer 79. In this example, the encoded data segment 94 includesthirty-two bits, but may include more or less bits. The slicer 79disperses the bits of the encoded data segment 94 across the EC dataslices in a pattern as shown. As such, each EC data slice does notinclude consecutive bits of the data segment 94 reducing the impact ofconsecutive bit failures on data recovery. For example, if EC data slice2 (which includes bits 1, 5, 9, 13, 17, 25, and 29) is unavailable(e.g., lost, inaccessible, or corrupted), the data segment can bereconstructed from the other EC data slices (e.g., 1, 3 and 4 for a readthreshold of 3 and a width of 4).

FIG. 6 is a flowchart illustrating an example of storing data as encodeddata slices. The method begins with step 102 were a processing modulereceives a store data object message from one or more of a user device,a storage integrity processing unit, a dispersed storage (DS) processingunit, a DS managing unit, and a DS unit. The store data object messageincludes one or more of a data object name, a data object size, a datatype, a data object, an input metadata, a priority indicator, a securityindicator, a performance indicator, a user device identifier (ID), andinput requirements.

The method continues at step 104 where the processing module determinesmetadata regarding data of the data object. Such metadata summarizesattributes of the data object including identification of priority data,identification of non-priority data, a data type, a data size indicator,and storage requirements. Such a determination may be based on one ormore of a vault lookup, a command, a message, a predetermination, a dataobject analysis, the data object name, the data object size, a datatype, the data object, the input metadata, the priority indicator, thesecurity indicator, the performance indicator, the input requirements,and retrieval of a metadata file that is linked to a data objectindicating previous metadata.

The method continues at step 106 where the processing module determinesrequirements associated with storing the data object including one ormore of expected access frequency, priority, security, performance,access latency, and reliability. Such a determination may be based onone or more of the metadata, a metadata analysis, a vault lookup, acommand, a message, a predetermination, a data object analysis, the dataobject name, the data object size, a data type, the data object, theinput metadata, the priority indicator, the security indicator, theperformance indicator, and the input requirements. For example, theprocessing module determines that the access latency requirement isslower than average and the reliability requirement is higher thanaverage when the performance indicator indicates that more reliabilityis favored over faster access latency.

The method continues at step 108 where the processing module determinesDS unit performance history with regards to DS units of a dispersedstorage network (DSN) memory. Such DS unit performance history includesone or more of history of reliability, availability, access latency,bandwidth utilization, security performance, and cost. Such adetermination may be based on one or more of a vault lookup, a command,a message, a predetermination, and DS unit query. The processing moduledetermines the DS unit performance history for a plurality of DS unitswhere the plurality of DS units are candidates to be included in a DSunit storage set.

The method continues at step 110 where the processing module determinesDS unit estimated performance. Such DS unit estimated performanceincludes one or more of a performance based indication regarding storageof a data segment as a set of encoded data slices, an estimation ofreliability, estimated availability, estimated access latency, estimatedbandwidth utilization, estimated security performance, estimated cost,and estimated availability. Such a determination may be based on one ormore of a vault lookup, a command, a message, a predetermination, a DSunit query, the DS unit performance history, and an estimationalgorithm. For example, the processing module determines that DS unitestimated access latency performance is lower than average when the DSunit access latency performance history was lower than average. Asanother example, the processing module determines the DS unit estimatedreliability to be lower than average when the DS unit reliabilityhistory was lower than average. As yet another example, the processingmodule determines a performance based indication regarding storage of adata segment as a set of encoded data slices as the DS unit estimatedperformance.

The method continues at step 112 where processing module determineserror coding dispersal storage function parameters (e.g., operationalparameters). Such parameters may include one or more of a pillar widthn, a read threshold, a decode threshold k, a write threshold, anencode/decode algorithm, and an encryption method. Such a determinationmay be based on one or more of a vault lookup, a command, a message, apredetermination, a DS unit query, the DS unit performance history, theDS unit estimated performance, the requirements, the metadata, andinformation received in the store data object message. For example, theprocessing module determines to utilize a relatively large number of DSunits in the storage set (e.g., a higher pillar width n) and a lowerdecode threshold k when the processing module determines that the DSunits are less reliable than average and the requirements include higherthan average reliability. As another example, the processing moduledetermines to utilize a relatively small number of DS units in the DSunit storage set and a higher decode threshold k when the processingmodule determines that the DS units are more reliable than average andthe requirements include greater efficiency of storage.

The method continues at step 114 where the processing module determinesthe DS unit storage set based on one or more of an encoded data slice,an associated slice name, a vault lookup, identification of a slice namerange to be rebuilt, a second DS unit identifier associated with the DSunit affiliated with the associated slice name, a virtual dispersedstorage network (DSN) address to physical location table lookup, acommand, a message, a predetermination, a DS unit query, the errorcoding dispersal storage function parameters, the DS unit performancehistory, the DS unit estimated performance, the requirements, themetadata, and the information received in the store data object message.Note that the processing module may re-determine the error codingdispersal storage function parameters when DS units that meet therequirements are not available. Alternatively, the processing module maydetermine the DS unit storage set prior to determining the error codingdispersal storage function parameters.

The method continues at step 116 where the processing module dispersedstorage error encodes the data to produce a plurality of sets of encodeddata slices in accordance with the error coding dispersal storagefunction parameters. Next, the processing module appends one or more ofthe metadata, the requirements, the DS unit performance history, the DSunit estimated performance, a source name, a slice name, and the errorcoding dispersal storage function parameters to one or more of datasegments created from the data object prior to encoding and slicing thedata segment to enable subsequent re-creation of the data object.Alternatively, the processing module creates a metadata file thatincludes one or more of the metadata, the requirements, the DS unitperformance history, the DS unit estimated performance, the source name,the slice name, and the error coding dispersal storage functionparameters. Next, the processing module stores the metadata file the DSNmemory as encoded metadata slices to enable subsequent retrieval andre-creation of the data object. The method at step 118 continues wherethe processing module sends the plurality of sets of encoded data slicesto DS units of the DS unit storage set for storage therein.

FIG. 7 is a flowchart illustrating an example of re-storing data asencoded data slices, which includes some similar steps to FIG. 6. Themethod begins with step 120 where a processing module determines adispersed storage (DS) unit storage set based on one or more of a dataobject name, a user identifier (ID), where a sequence left off lasttime, a directory entry, a storage location table lookup, a command, amessage, a predetermination, and a query. As a specific example, theprocessing module determines a DS unit storage set 457 based on thestorage location table lookup corresponding to a data object foo.txt,wherein foo.txt is a next entry of a directory entry.

The method continues with steps 104-110 of FIG. 6 and then continueswith step 122 where the processing module determines whether to changethe DS unit storage set and/or to change error coding dispersal storagefunction parameters (e.g., operational parameters) based on a comparisonof the DS unit estimated performance to the requirements. For example,the processing module compares a performance based indication (e.g., theDS unit estimated performance based on the DS unit performance history)with a performance threshold of the requirements. Note that the DS unitestimated performance may have changed since encoded data slices wereoriginally stored to the DS unit storage set. Further note that therequirements may have changed since the slices were originally stored tothe DS unit storage set. The processing module determines to change theDS unit storage set and/or the error coding dispersal storage functionparameters when the processing module determines that the comparison ofthe DS unit estimated performance to the requirements is unfavorable.

For example, the processing module determines to change the DS unitstorage set when a DS unit estimated reliability level is below arequired reliability level. As another example, the processing moduledetermines to change the error coding dispersal storage functionparameters when a DS unit estimated access latency time is above arequired access latency. As yet another example, the processing moduledetermines to change the DS unit storage set and the error codingdispersal storage function parameters when the DS unit estimatedreliability level is below the required reliability level and the DSunit estimated access latency time is above the required access latency.

The method repeats back to step 120 when the processing moduledetermines not to change the DS unit storage set and/or the error codingdispersal storage function parameters (e.g., the performance basedindication compares favorably with the performance threshold). Themethod continues to step 112 of FIG. 6 when the processing moduledetermines to change the DS unit storage set and/or the error codingdispersal storage function parameters (e.g., the performance basedindication compares unfavorably with the performance threshold).

The method continues at step 124 where the processing module retrieves aset of encoded data slices from the DS unit storage set by sending aread encoded data slice message to at least a read threshold number ofDS units of the DS unit storage set and receiving at least a decodethreshold number of encoded data slices of the set of encoded dataslices. The method continues at step 126 where the processing moduledispersed storage error decodes the at least the decode threshold numberof encoded data slices of the set of encoded data slices in accordancewith the error coding dispersal storage function parameters to reproducea data segment as a reproduced data segment.

The method continues at step 128 where the processing module adjusts theerror coding dispersal storage function parameters based on theunfavorable comparison of the performance based indication with theperformance threshold to produce performance adjusted error codingdispersal storage function parameters. Such adjusting includes one ormore of determining desired error coding dispersal storage functionparameters based on the performance threshold and the performance basedindication, determining a difference between desired error codingdispersal storage function parameters and the error coding dispersalstorage function parameters to produce a parameters difference, andadjusting the error coding dispersal storage function parameters basedon the parameters difference, wherein determining at least one of thedesired error coding dispersal storage function parameters and theparameters difference is based on at least one of a set of DS units, theerror coding dispersal storage function parameters, a vault lookup, acommand, a message, a predetermination, a DS unit query, a historical DSunit performance level, an estimated DS unit performance level, storagerequirements, security requirements, and metadata.

For example, the processing module adjusts the error coding dispersalstorage function parameters by increasing a difference between a decodethreshold and a pillar width and adjusting an encoding matrix inaccordance with the increasing of the difference when the performancebased indication compares unfavorably with the performance threshold asa result of under-performance. As another example, the processing moduleadjusts the error coding dispersal storage function parameters bydecreasing a difference between a decode threshold and a pillar widthand adjusting an encoding matrix in accordance with the increasing ofthe difference when the performance based indication comparesunfavorably with the performance threshold as a result ofover-performance. As yet another example, the processing module adjuststhe error coding dispersal storage function parameters to include adifferent encryption algorithm when the security requirements havechanged. Next, the processing module saves the performance adjustederror coding dispersal storage function parameters by storing theperformance adjusted error coding dispersal storage function parametersin a local memory and/or as encoded parameters slices in a dispersedstorage network (DSN) memory.

The method continues at step 130 where the processing module determinesa new DS unit storage set based on one or more of a vault lookup, acommand, a message, a predetermination, a DS unit query, the errorcoding dispersal storage function parameters, the performance adjustederror coding dispersal storage function parameters, the DS unit storageset, the DS unit performance history, the DS unit estimated performance,the requirements, the metadata, and information received in the storedata object message. Next, processing module saves the new DS unitstorage set selection by storing the new DS unit storage set selectionin a local memory and/or as encoded DS unit selection slices in the DSNmemory.

The method continues at step 132 where the processing module encodes thereproduced data segment in accordance with the performance adjustederror coding dispersal storage function parameters to produce a secondset of encoded data slices. Next, the processing module selects astorage set of encoded data slices from the set of encoded data slicesand the second set of encoded data slices based on a difference betweenthe performance adjusted error coding dispersal storage functionparameters and the error coding dispersal storage function parameters.Such selecting of the storage set of encoded data slices includes atleast one of selecting the set of encoded data slices and selecting atleast one encoded data slice of the second set of encoded data slices.Next, processing module updates a storage location table to associate acorresponding slice name of a set of slice names with a correspondingencoded data slice of the storage set of encoded data slices tofacilitate subsequent retrieval.

Alternatively, retrieval may be accomplished by a lookup of originallocations of original encoded data slices and extend address ranges ofthe original locations to provide locations of newer encoded data slicesassociated with the original encoded data slices. Note that the firstdecode threshold number k of encoded data slices of the second set ofencoded data slices specify the reproduced data segment in its originalformat when an encoding matrix includes a unity sub-matrix. Further notethat remaining n-k encoded data slices of the second set of encoded dataslices specify parity information that may be utilized in subsequentretrieval process to correct for errors when the first k encoded dataslices are not successfully retrieved. Further note that the first setof encoded data slices are included in the second set of encoded dataslices when the second set of encoded data slices are produced byencoding the reproduced data segment with an encoding matrix that isidentical to the encoding matrix utilized to produce the first set ofencoded data slices with the exception that one or more rows are addedwhich result in generation of further parity slices. For example, theprocessing module produces one new slice (e.g., slice number 17) in thesecond set of encoded data slices when the error coding dispersalstorage function parameters include a pillar width of 16, and a decodethreshold of 10, and the performance adjusted error coding dispersalstorage function parameters include a pillar width of 17 and a decodethreshold of 10. The method continues at step 134 where the processingmodule outputs each of the encoded data slices of the storage set ofencoded data slices that is selected from the second set of encoded dataslices to the new DS unit storage set of the DSN memory for storagetherein.

FIG. 8A is a diagram illustrating data segmentation of data thatincludes a data object 136, a segment set 138, a segment set R 140, anda segment set P 142. The data object 136 includes data wherein the datacomprises non-priority data 1-4 and priority data 1-3. Note thatpriority data may be more important with regards to subsequentutilization of the data object as compared to the non-priority data. Inan example, priority data includes header information, codecinformation, and base frames associated with a compressed video file andnon-priority data includes different video frames of the compressedvideo file.

The data object 136 may be stored in a dispersed storage network (DSN)memory as a plurality of sets of encoded data slices, wherein each setof encoded data slices corresponds to encoding a plurality of datasegments of the data object 136. The segment set 138 represents datasegments 1-8, wherein each of the data segments are substantially thesame size created as a result of segmentation of the data object 136without regard to non-priority data or priority data. Alternatively, orin addition to, the data object 136 may be stored in the DSN memory as aplurality of sets of priority encoded data slices corresponding toencoding a plurality of priority data segments of the data object 136.The segment set R 140 represents priority data segments 2-4 and 6-8,wherein each of the data segments are substantially the same size ascorresponding data segments 2-4, and 6-8 that contain priority data ofthe plurality of data segments created as a result of segmentation ofthe data object 136 to capture priority data 1-3. Alternatively, or inaddition to, the segment set P 142 represents priority data 1-3, whereineach of the data segments are substantially the same size ascorresponding priority data 1-3. A method of storing data segments andpriority data segments is discussed in greater detail with reference toFIG. 9. A method of retrieving data segments and priority data segmentsis discussed in greater detail with reference to FIG. 10.

Note that the storing of the plurality of sets of priority encoded dataslices provides a priority data retrieval reliability improvement whenthe priority data segments are stored in addition to data segmentscorresponding to priority data. Further note that the storing of theplurality of sets of priority encoded data slices provides a prioritydata retrieval reliability improvement when the priority data is storedas priority data segments rather than as data segments and when errorcoding dispersal storage function parameters associated with theencoding of the priority data segments provides more reliable storage ascompared to error coding dispersal storage function parametersassociated with the encoding of data segments. Such differences in errorcoding dispersal storage function parameters are discussed in greaterdetail with reference to FIG. 8B.

FIG. 8B is a diagram illustrating an example of encoding data segmentsthat includes a data object 136, a segment set 138, and a segment set P142 of FIG. 8A and a plurality of sets of encoded data slices 139 and aplurality of sets of priority encoded data slices 143. Each data segmentof the segment set 138 is dispersed storage error encoded in accordancewith first error coding dispersal storage function parameters to producethe plurality of sets of encoded data slices 139. For example, datasegment 3 is dispersed storage error encoded to produce a set of encodeddata slices 3_(—)1-3_(—)4, when a pillar width is 4 of the first errorcoding dispersal storage function parameters. Each priority data segmentof the segment set P 142 is dispersed storage error encoded inaccordance with second error coding dispersal storage functionparameters to produce the plurality of sets of priority encoded dataslices 143. For example, data segment P2 is dispersed storage errorencoded to produce a set of encoded data slices P2_(—)1-P2_(—)8, when apillar width is 8 of the second error coding dispersal storage functionparameters.

Note that the first error coding dispersal storage function parametersmay be determined such that dispersed storage error encoding datasegments containing only non-priority data 1-4 of the segment set 138 toproduce encoded data slices of the plurality of sets of encoded dataslices 139 in accordance with the first error coding dispersal storagefunction parameters will result in a desired level of non-priority dataretrieval reliability. For example, first error coding dispersal storagefunction parameters that include a pillar width of 4 and a decodethreshold of 3 are selected to provide a desired level of non-prioritydata retrieval reliability. Note that improved retrieval reliability isprovided when a difference between a pillar width and a decode thresholdof the second error coding dispersal storage function parameters isgreater than a difference between a pillar width and a decode thresholdof the first error coding dispersal storage function parameters. Thesecond error coding dispersal storage function parameters may bedetermined such that dispersed storage error encoding priority datasegments containing priority data 1-3 of the segment set P 142 toproduce priority encoded data slices of the plurality of sets ofpriority encoded data slices 143 in accordance with the second errorcoding dispersal storage function parameters will result in a desiredlevel of priority data retrieval reliability. For example, second errorcoding dispersal storage function parameters that include a pillar widthof 8 and a decode threshold of 5 are selected to provide a higher levelof priority data retrieval reliability as compared to utilizing firsterror coding dispersal storage function parameters when the pillar widthis 4 and the decode threshold is 3.

FIG. 9 is a flowchart illustrating an example segmenting data, whichincludes some similar steps to FIG. 6. The method begins with steps102-114 of FIG. 6 and then continues with step 148 where a processingmodule encodes data into a plurality of sets of encoded data slices inaccordance with first error coding dispersal storage functionparameters. Note that the processing module may encode all data (e.g.,non-priority data and priority data) or the processing module encodesthe non-priority data alone. For example, the processing moduleidentifies non-priority data segments of the data and encodes thenon-priority data segments into the plurality of sets of encoded dataslices in accordance with the first error coding dispersal storagefunction parameters, wherein the first error coding dispersal storagefunction parameters include a first pillar width and a first decodethreshold optimized for data recovery (e.g., retrieving and decoding)speed and non-optimal for data recovery reliability. The methodcontinues at step 150 where the processing module outputs the pluralityof sets of encoded data slices to the first dispersed storage (DS) unitstorage set of a dispersed storage network (DSN) memory for storagetherein. Next, the processing module updates a storage location table toassociate the plurality of sets of encoded data slices with the data.

The method continues at step 152 where the processing module determineswhether to create priority data segments (e.g., and store them in theDSN memory as encoded data slices) based on one or more of requirements,metadata, a data object, a determination of the priority/non-prioritydata, which data segments from the original segment set include prioritydata, a command, a predetermination, a lookup, DS unit performancehistory, and DS unit estimated performance. For example, the processingmodule determines to create more data segments when the metadataidentifies priority data and when the performance history of the firstDS unit storage set is unfavorable as compared to the requirements(e.g., the DS unit reliability history or DS unit estimated reliabilityis below a threshold). The method ends at step 154 when the processingmodule determines not to create priority data segments. The methodcontinues to step 136 when the processing module determines to createpriority data segments.

The continues at step 156 where the processing module determines anapproach to create priority data segments based on one or more of therequirements, the data object, a determination of thepriority/non-priority data, a size of the priority data, a location ofthe priority data, which data segments from the original segment setinclude priority data, a command, a predetermination, a lookup, DS unitperformance history, and DS unit estimated performance. For example, theprocessing module determines to create a segment set R when the size ofthe priority data is substantially contained within a data segment ofthe original data segment set described above (e.g., without overlappingtwo or more data segments). Note that the processing module may utilizethe data segments of the original segment set to produce (e.g., copyfrom a temporary memory) the data segments of the segment set R. Asanother example, the processing module determines to create a segmentset P when the size of the priority data is substantially not containedwithin a data segment of the original data segment set (e.g.,substantially overlapping two or more data segments).

The method continues at step 158 where the processing module determinespriority data segments of the data. Such a determination includes atleast one of identifying content of the data having a desired prioritylevel, wherein data segments containing the content are identified asthe priority data segments, determining a desired relationship between adata recovery speed and a data recovery reliability, and determining thepriority data segments based on the desired relationship, receiving anindicator that identifies the priority data segments, and accessing atable regarding the data to identify the priority data segments. Forexample, the processing module copies the data segments from theoriginal segment set that correspond to priority data of the data objectwhen the approach is to form a segment set R of priority data segments.As another example, the processing module forms each priority datasegment from the priority data sections of the data object when theapproach is to form a segment set P of priority data segments thatreplicate the priority data. As yet another example, the processingmodule encodes video data into the plurality of sets of encoded dataslices in accordance with the first error coding dispersal storagefunction parameters, wherein the plurality of set of encoded data slicesis retrieved in response to a video on demand request and the processingmodule determines key frames of the video data as the priority datasegments.

The method continues at step 160 where the processing module savespriority data segment information to facilitate subsequent retrievalwhere the priority data segment information may include one or more of alocation of the priority data in the data object, a size of the prioritydata segments, a size of the priority data, and a number of prioritydata sections. For example, the processing module saves the prioritydata segment information as data appended to one or more of the prioritydata segments. As another example, the processing module saves thepriority data segment information as a separate data object in the DSNmemory as a plurality of sets of encoded priority data segmentinformation slices.

The method continues at step 162 where the processing module determinessecond error coding dispersal storage function parameters (e.g., secondoperational parameters) and a second DS unit storage set based on one ormore of the determined approach to create priority data segments, thepriority data segments, the requirements, the metadata, the DS unitperformance history, the DS unit estimated performance, the first DSunit storage set, the first error coding dispersal storage functionparameters, the data object, a command, a predetermination, and a vaultlookup. For example, the processing module determines the second errorcoding dispersal storage function parameters and the second DS unitstorage set to be substantially identical to the first error codingdispersal storage function parameters and the first DS unit storage setwhen a nominal level of improved retrieval reliability is required. Asanother example, the processing module determines the second errorcoding dispersal storage function parameters and the second DS unitstorage set to be substantially different from the first error codingdispersal storage function parameters and the first DS unit storage set.As a specific example, the processing module determines the second errorcoding dispersal storage function parameters to include a second pillarwidth and a second decode threshold optimized for data recoveryreliability and non-optimal for data recovery speed when an improvedlevel of retrieval reliability is required.

The method continues at step 164 where the processing module encodes thepriority data segments in accordance with the second error codingdispersal storage function parameters to produce a plurality of sets ofpriority encoded data slices. The method continues at step 166 where theprocessing module outputs the plurality of sets of priority encoded dataslices to the second DS unit storage set of the DSN memory for storagetherein. Next, the processing module updates the storage location tableto associate the plurality of sets of priority encoded data slices withthe priority data segments. The method repeats back to step 152 topotentially create more priority and/or redundant data segments.

FIG. 10 is a flowchart illustrating an example of re-creating data,which includes similar steps to FIG. 6 and FIG. 9. The method beginswith step 168 where a processing module receives a retrieve data objectmessage (e.g., from any one of a user device, a storage integrityprocessing unit, a dispersed storage (DS) processing unit, a DS managingunit, and a DS unit). The retrieve data object message includes one ormore of a data object name, a data object size, a data type, an inputmetadata, a priority data indicator, a priority indicator, a securityindicator, a performance indicator, and input requirements. The methodcontinues at steps 112-114 of FIG. 6 and then continues with step 170where the processing module retrieves a set of encoded data slices froma first DS unit storage set of a dispersed storage network (DSN) memory,wherein a data segment was encoded in accordance with first error codingdispersal storage function parameters to produce the set of encoded dataslices.

The method continues at step 172 where the processing module determineswhether the set of encoded data slices are associated with a datasegment or a priority data segment to facilitate re-creating datasegments. Such a determination may be based on one or more of accessinga storage location table to identify the set of encoded data slices withthe data, accessing the storage location table to identify the secondset of encoded data slices, the first error coding dispersal storagefunction parameters, an origin of retrieval of the set of encoded dataslices, a command, a message, and a query. The processing module decodesthe set of encoded data slices in accordance with the first error codingdispersal storage function parameters to produce the data segment whenthe data segment is not the priority data segment.

The processing module determines whether to decode the set of encodeddata slices in accordance with the first error coding dispersal storagefunction parameters or to decode a second set of encoded data slices inaccordance with second error coding dispersal storage functionparameters when the data segment is the priority data segment, whereinthe data segment was encoded in accordance with the second error codingdispersal storage function parameters to produce the second set ofencoded data slices, wherein the first error coding dispersal storagefunction parameters include a first pillar width and a first decodethreshold optimized for data recovery speed and non-optimal for datarecovery reliability and the second error coding dispersal storagefunction parameters include a second pillar width and a second decodethreshold optimized for data recovery reliability and non-optimal fordata recovery speed.

Such determining whether to decode the set of encoded data slices or thesecond set of encoded data slices includes at least one of determining adesired relationship between the data recovery speed and the datarecovery reliability, decoding the set of encoded data slices when thedesired relationship compares favorably to a relationship threshold, anddecoding the second set of encoded data slices when the desiredrelationship compares unfavorably to the relationship threshold,receiving an indicator that indicates whether to decode the set ofencoded data slices or the second set of encoded data slices, andaccessing a table based on identity of the data segment to determinewhether to decode the set of encoded data slices or the second set ofencoded data slices. For example, the processing module retrieves theset of encoded data slices in response to a video on demand request anddetermines whether the data segment corresponds to a key frame of videodata. Next, the processing module identifies the data segment as apriority data segment and determines to decode the second set of encodeddata slices when the data segment corresponds to the key frame (e.g.,priority data).

The method continues at step 174 where the processing module determinesif enough data segments have been re-created. Such a determination maybe based on one or more of a comparison of the number of re-created datasegments to a number of data segments of the data object, a vaultlookup, a command, a message, a predetermination, a priority dataindicator, metadata, and information received in the retrieve dataobject message. For example, the processing module determines thatenough data segments have been received when all of the data segmentshave been received and metadata in the retrieve data object messageindicated that all of the data segments are required for this retrievaltransaction. As another example, the processing module determines thatenough data segments have not been received when all of the datasegments have not been received and metadata in the retrieve data objectmessage indicates that all of the data segments are required for thisretrieval transaction.

As yet another example, the processing module determines that enoughdata segments have been received when all of the data segments have notbeen received, data segments containing priority data have been receivedthat contain all of the priority data, and metadata in the retrieve dataobject message indicates that only the priority data is required forthis retrieval transaction. As a further example, the processing moduledetermines that not enough data segments have been received when all ofthe data segments have not been received, not enough data segmentscontaining priority data have been received such that at least some ofthe priority data has not been retrieved, and metadata in the retrievedata object message indicates that only the priority data is requiredfor this retrieval transaction.

The method branches to step 180 when the processing module determinesthat enough data segments have not been received. The method continuesto step 176 when the processing module determines that enough datasegments have been received. The method continues at step 176 where theprocessing module re-creates priority data and/or non-priority data byaggregating the successfully re-created data segments and priority datasegments. The method continues at step 178 where the processing modulesends the data to the requester.

The method continues at step 180 where the processing module determineswhether all of the priority data segments have been recovered (e.g.,retrieving a segment set R and/or retrieving a segment set P anddecoding the segments) based on one or more of previously recovered datasegments, a vault lookup, a message, a command, and a list of segmentsets tried. The method branches to step 184 when the processing moduledetermines that all priority data segments have not been recovered. Themethod continues to step 182 when the processing module determines thatall priority data segments have been recovered. The method continues atstep 182 where the processing module sends a fail message to therequester and/or the DS managing unit.

The method continues at step 184 where the processing module determinesadditional priority data segments to try next based on one or more ofpreviously recovered priority data segments, a vault lookup, a message,a command, and a list of segment sets recovered. For example, theprocessing module determines to recover an additional priority datasegment set after trying to recover the original data segment set. Asanother example, the processing module determines to try segment set Pafter trying to retrieve the original data segment set and a prioritysegment set. The method continues at step 162 of FIG. 9.

The method continues at step 190 where the processing module sends aretrieve slice command to at least a DS unit of the second DS unitstorage set to retrieve a second set of encoded data slices from asecond DS unit storage set of the DSN memory. The processing modulereceives priority encoded data slices from at least one DS unit of thesecond DS unit storage set in response to the retrieve slice command.The processing module retrieves at least a read threshold number ofencoded priority data slices from the second DS unit storage setcorresponding to each of the desired priority data segments. The methodcontinues at step 192 where the processing module dispersed storageerror decodes the second set of encoded data slices in accordance withthe second error coding dispersal function parameters to produce thepriority data segment as a recovered data segment. The method repeatsback to step 172 to fully re-create data segments and determine ifenough data segments have been re-created to facilitate aggregating datasegments to reproduce the data.

FIG. 11 is a flowchart illustrating an example of caching encoded dataslices, which includes many steps similar to FIG. 6. The method beginswith steps 102, 108, 112, 114, 116, 118 of FIG. 6 and then continueswith step 194 where a processing module determines which encoded dataslices to cache based on one or more of a comparison of dispersedstorage (DS) unit access latency history to a threshold, DS unitperformance history, a DS unit storage set, error coding dispersalstorage function parameters, metadata, requirements, a vault lookup, acommand, and information received in the store data object message. Forexample, the processing module determines the slices to cache when theslices correspond to the DS units with an access latency timeperformance that is above a threshold. Alternatively, the processingmodule skips step 108 to enable always caching the encoded data slicesnot subject to DS unit performance history.

The method continues at step 196 where the processing module caches theslices that are to be cached as cached encoded data slices. For example,the processing module determines to temporarily store the encoded dataslices in one or more memories of one or more of a DS processing unit, auser device, a DS managing unit, a storage integrity processing unit,and a DS unit. The processing module determines cache location based onone or more of a candidate cache memory list, memory availabilityindicator, a size indicator of the encoded data slices to cache, aperformance indicator of cache memory, requirements, the DS unit storageset, the error coding dispersal storage function parameters, a vaultlookup, a command, a predetermination, and information received in thestore data object message. As a specific example, the processing modulecaches the encoded data slices in a memory associated with a DSprocessing unit such that the encoded data slices are readily availablefor retrieval with a relatively low access latency. Alternatively, or inaddition to, the processing module facilitates storage of the encodeddata slice in temporary memory to produce a temporarily stored encodeddata slice when estimated DS unit performance level compares unfavorablywith a performance threshold, wherein the temporarily stored encodeddata slice is retrieved in response to a retrieval request of theencoded data slice when confirmation of a DS unit storing the encodeddata slice has not been received.

The method continues at step 198 where the processing module updates astorage location table (e.g., a virtual dispersed storage network (DSN)address to physical location table) to associate a slice name with atemporary memory identifier (ID) of the temporary memory. For example,the processing module updates the virtual DSN address to physicallocation table to include pillars 1-9 as stored in DS units 1-9, pillar10 as stored in a DS processing unit cache memory, and pillars 11-16 isstored in DS units 11-16 when the pillar width is 16 and the readthreshold is 10.

The method continues at step 200 where the processing module determineswhether the slices are available for retrieval from the DS units basedon receiving a response from the DS units, wherein the responseindicates that the encoded data slices are available for retrieval. Themethod advances to step 202 when the processing module determines thatthe slices are available for retrieval from the DS units. The methodcontinues at step 202 where the processing module updates a storagelocation table (e.g., the virtual DSN address to physical locationtable) to associate slice names of the encoded data slices with DS unitIDs of the DS unit storage set and to delete an association of the slicenames with the temporary memory ID of the temporary memory. The methodcontinues at step 204 where the processing module facilitates deletingof the cached encoded data slices from the temporary memory.

In an example of operation of a corresponding retrieval sequence, theprocessing module receives a retrieve data object message from arequester and determines DS unit locations based on a lookup of thevirtual DSN address to physical location table. Note that the table mayindicate a combination of DS units and or cache memory locations for atleast some of encoded data slices to be retrieved. The processing moduledetermines error coding dispersal storage function parameters based on avault lookup. The processing module retrieves the encoded data slicesfrom the determined locations dispersed storage error decodes theencoded data slices to produce data segments in accordance with theerror coding dispersal storage function parameters. The processingmodule aggregates the data segments to produce the data object. Theprocessing module sends the data object to the requester.

FIGS. 12A-C are schematic block diagrams of an embodiment of a dispersedstorage network (DSN) that includes a dispersed storage (DS) processingunit 16 of FIG. 1, the network 24 of FIG. 1, and the dispersed storagenetwork (DSN) memory 22 of FIG. 1. The DSN memory 22 includes a set ofDS units 36 of FIG. 1. Hereafter, a DS unit 36 may be interchangeablyreferred to as a storage unit and the set of DS units 36 may be referredto interchangeably as a set of storage units. Each DS processing unitincludes a memory A and the DS processing 34 of FIG. 1. Each memory Amay be implemented using the main memory 54 of FIG. 2. Hereafter, thememory A may be interchangeably referred to as a local memory. The DSNfunctions to cache a rebuilt encoded data slice via a series of steps.

FIG. 12A illustrates initial steps of the caching the rebuilt encodeddata slice. As a specific example, the DS processing 34 executes arebuild function to produce the rebuilt encoded data slice for storagein a targeted storage unit of the DSN. For instance, the DS processing34 retrieves, via the network 24, retrieved slices 201 that includes adecode threshold number of encoded data slices of a set of encoded dataslices, where a data segment was dispersed storage error encoded toproduce a set of encoded data slices 1-n. Having retrieved the decodethreshold number of encoded data slices, the DS processing 34 dispersedstorage error decodes the decode threshold number of encoded data slicesto reproduce the data segment and dispersed storage error encodes thereproduced data segment to produce a rebuilt slice 2 as the rebuiltencoded data slice when a corresponding slice 2 of the set of encodeddata slices 1-n is not available. The slice 2 may not be available dueto one or more of a failure of DS unit 2, a failure of connectivitybetween network 24 and the DS unit 2, and a failure of a memory of DSunit 2.

The memory A includes a slice location table 203. The slice locationtable includes one or more fields from a list of fields that includes aslice identifying field (e.g., slice identifier 2), a storage locationfield (e.g., DS unit 2), and a targeted storage unit status field (e.g.,DS unit 2). The DS processing 34 may access the slice location table todetermine a storage location associated with each encoded data slice ofthe set of encoded data slices 1-n.

FIG. 12B illustrates further steps of the caching the rebuilt encodeddata slice. As a specific example, when the targeted storage unit (e.g.,DS unit 2) is currently unavailable (e.g., as indicated by a statusindicator and/or an access response), the DS processing 34 stores therebuilt encoded data slice in the local memory. For instance, the DSprocessing stores rebuilt slice 2 in memory A. Having stored the rebuiltencoded data slice in the local memory, the DS processing 34 recordsstorage of the rebuilt encoded data slice in the local memory. Forexample, the DS processing 34 creates an entry in the slice locationtable 203 for the rebuilt encoded data slice 2 to indicate that therebuilt encoded data slice 2 is stored in memory A (MEM A). Havingrecorded the storage of the rebuilt encoded data slice in the localmemory, the DS processing 34 records that the targeted storage unit iscurrently unavailable. For instance, the DS processing 34 updates astorage unit status indicator within the memory A to indicate that theDS unit 2 is unavailable.

As another specific example, when the targeted storage unit is currentlyavailable, the DS processing 34 stores the rebuilt encoded data slice inthe local memory and records the storage of the rebuilt encoded dataslice in the local memory. As yet another specific example, when thetargeted storage unit is currently available, the DS processing 34 sendsthe rebuilt encoded data slice to the targeted storage unit. Forinstance, the DS processing 34 sends, via the network 24, the rebuiltencoded data slice 2 to DS unit 2 for storage when DS unit 2 isavailable. Having sent the rebuilt encoded data slice to the targetedstorage unit, the DS processing 34 records storage of the rebuiltencoded data slice by the targeted storage unit. For example, the DSprocessing 34 updates the slice location table 203 to indicate that therebuilt encoded data slice 2 is stored in a DS unit 2.

As a further specific example, when the targeted storage unit wasunavailable and becomes available, the DS processing 34 sends therebuilt encoded data slice to the targeted storage unit. For instance,the DS processing 34 sends, via the network 24, the rebuilt encoded dataslice 2 to the DS unit 2 for storage. Having sent the rebuilt encodeddata slice to the targeted storage unit, the DS processing 34 updatesthe recording the storage of the rebuilt encoded data slice to includestorage by the targeted storage unit and maintains storage of therebuilt data slice in the local memory. Having stored the rebuiltencoded data slice in both the local memory and the storage unit, the DSprocessing 34 updates the recording to include storage of the rebuiltencoded data slice in both of the local memory and the targeted storageunit.

FIG. 12C illustrates still further steps of the caching the rebuiltencoded data slice. As a specific example, when the targeted storageunit is currently unavailable and a data retrieval request is received,the DS processing 34 retrieves the rebuilt encoded data slice 2 from thelocal memory and sends a read response to a requesting entity, where theread response includes a copy of the rebuilt encoded data slice storedin the local memory. For instance, the DS processing 34 retrieves therebuilt encoded data slice 2 from the memory A, generates a readresponse to include the rebuilt encoded data slice 2, and sends the readresponse to the requesting entity.

As another specific example, when the targeted storage unit is currentlyunavailable and the data retrieval request is received, the DSprocessing 34 retrieves at least a decode threshold number of encodeddata slices that includes retrieving the rebuilt encoded data slice fromthe memory A and retrieving other encoded data slices of the at least adecode threshold number of encoded data slices as other retrieve slices205 from storage units of the DSN memory 22. Having recovered the decodethreshold number of encoded data slices, the DS processing 34 dispersedstorage error decodes the decode threshold number of encoded data slicesto produce a recovered data segment 207.

FIG. 12D is a flowchart illustrating an example of caching rebuiltencoded data slices, which includes many similar steps to FIGS. 6, 7,and 11. The method begins with step 206 where a processing moduledetermines to rebuild data where the data may be at least a portion of adata object stored as encoded data slices in a dispersed storage network(DSN) memory. Such a determination may be based on one or more ofdetection of a missing slice, detection of a corrupted slice, detectionof a tampered slices, detection of a failed memory device, detection ofa failed DS unit, detection of a failed site, a message, a command, anda DS unit query. For example, the processing module receives a messagefrom a DS unit indicating that one of four hard drive memories hasfailed and has been replaced with a new hard drive memory. The methodcontinues with steps 114, 108, and 112 of FIG. 6. The method continueswith steps 124-126 of FIG. 7 to reproduce a data object corresponding toat least an encoded data slice to be rebuilt. Note that the processingmodule may obtain an encoded data slice and an associated slice name forstorage in a DS unit and/or a temporary memory wherein the obtaining theencoded data slice comprises at least one of receiving the encoded dataslice, creating the encoded data slice, rebuilding the encoded dataslice from a set of associated encoded data slices, and receiving theencoded data slice as a rebuilt encoded data slice that was rebuilt fromthe set of associated encoded data slices.

The method continues at step 208 where the processing module dispersedstorage error encodes a data segment associated with the portion of thedata object to be rebuilt to produce an encoded data slice as a rebuiltslice in accordance with error coding dispersal storage functionparameters. The method continues at step 210 where the processing modulesends the encoded data slice to an associated DS unit of a DS unitstorage for storage therein. Alternatively, the processing module sendsthe encoded data slice through the associated DS unit subsequent to step196 where the processing module caches the rebuilt slice.

The method continues at step 212 where the processing module determinesa temporary memory based on at least one of a slice name, a size of theencoded data slice, a comparison of DS unit access latency history to athreshold, a DS unit performance history, the DS unit storage set, theerror coding dispersal storage function parameters, metadata,requirements, a vault lookup, a command, and information obtainedassociated with the encoded data slice, wherein the temporary memoryincludes one or more of another DS unit, local memory, cache memory, andmain memory. The method continues at step 196 of FIG. 11 to cache therebuilt slice in the temporary memory. The method continues at step 198of FIG. 11.

The method continues at step 214 where the processing module determineswhether the slice is available for retrieval from the DS unit based onreceiving a response from the DS unit, wherein the response indicatesthat the encoded data slice is available for retrieval. The methodadvances to step 216 when the processing module determines that theslice is available for retrieval from the DS unit. The method continuesat step 216 where the processing module updates a storage location table(e.g., the virtual DSN address to physical location table) to associatethe slice name of the encoded data slice with a DS unit ID of the DSunit and to delete an association of the slice name with the temporarymemory ID of the temporary memory. The method continues at step 217where the processing module facilitates deleting of the cached encodeddata slice from the temporary memory.

FIG. 12E is a flowchart illustrating an example of caching rebuiltencoded data slices. In particular, a method is presented for use inconjunction with one or more functions and features described inconjunction with FIGS. 1-5, 12A-D, and also FIG. 12E. The methodcontinues at step 209 where a processing module of a computing device ofa dispersed storage network (DSN) executes a rebuild function to producea rebuilt encoded data slice for storage in a targeted storage unit ofthe DSN. As a specific example, the processing module retrieves a decodethreshold number of encoded data slices of a set of encoded data slices,where a data segment was dispersed storage error encoded to produce aset of encoded data slices, dispersed storage error decodes the decodethreshold number of encoded data slices to reproduce the data segment,and dispersed storage error encodes the reproduced data segment toproduce the rebuilt encoded data slice. The method branches to step 215when the targeted storage unit is currently unavailable. The methodcontinues to step 211 when the targeted storage unit is currentlyavailable.

When the targeted storage unit is currently available, the methodcontinues at step 211 where the processing module sends the rebuiltencoded data slice to the targeted storage unit. The sending may includerecording storage of the rebuilt encoded data slice by the targetedstorage unit. As a specific example of the recording, the processingmodule creates an entry in a slice location table for the rebuiltencoded data slice, where the slice location table includes one or morefields from a list of fields that includes a slice identifying field, astorage location field, and a targeted storage unit status field.

When the targeted storage unit is currently available, the method maycontinue at step 213 where the processing module stores the rebuiltencoded data slice in a local memory of the computing device. Whenstoring the rebuilt encoded data slice in a local memory, the processingmodule records the storage of the rebuilt encoded data slice in thelocal memory.

When the targeted storage unit is currently unavailable, the methodcontinues at step 215 where the processing module stores the rebuiltencoded data slice in the local memory of the computing device. Thestoring includes recording storage of the rebuilt encoded data slice inthe local memory and recording that the targeted storage unit iscurrently unavailable. When the targeted storage unit is currentlyunavailable and a data retrieval request is received, the methodcontinues at step 219 where the processing module sends a read responsethat includes a copy of the rebuilt encoded data slice stored in thelocal memory.

When the targeted storage unit was unavailable and becomes available,the method continues at step 221 where the processing module sends therebuilt encoded data slice to the targeted storage unit. The sendingincludes one or more of updating the recording the storage of therebuilt encoded data slice to include storage by the targeted storageunit, maintaining storage of the rebuilt data slice in the local memory,and updating the recording to include storage of the rebuilt encodeddata slice in both of the local memory and the targeted storage unit.

The method described above in conjunction with a processing module canalternatively be performed by other modules of a dispersed storagenetwork or by other devices. In addition, at least one memory sectionthat stores operational instructions can, when executed by one or moreprocessing modules of one or more computing devices of a dispersedstorage network (DSN), cause the one or more computing devices toperform any or all of the method steps described above.

FIGS. 13A-C are schematic block diagrams of another embodiment of adispersed storage network (DSN) that includes the dispersed storage (DS)processing unit 16 of FIG. 1, the network 24 of FIG. 1, and the DSNmemory 22 of FIG. 1. The DS processing unit 16 includes the DSprocessing 34 of FIG. 1 and one or more cash memories 1-M. Each cachememory may be implemented utilizing the main memory 54 of FIG. 2.Hereafter, the cache memories 1-M may be referred to interchangeably asthe cache memory. The DSN memory 22 includes a set of DS units 1-n. EachDS unit may be implemented utilizing the DS unit 36 of FIG. 1.Hereafter, each DS unit may be referred to interchangeably as a storageunit.

The DSN functions to utilize cached encoded data slices, where a dataobject 222 is dispersed storage error encoded to produce a plurality ofsets of encoded data slices and where a write operation includestemporarily storing encoded data slices of the plurality of sets ofencoded data slices in the cache memory. As a specific example, the DSprocessing 34 partitions the data object 222 into M data segments anddispersed storage error encodes each data segment to produce M sets ofencoded data slices. The M sets of encoded data slices includes a firstset of encoded data slices that includes the encoded data slices 1-1,2-1, through n-1; a second set of encoded data slices that includes theencoded data slices 1-2, 2-2, through n-2; through an Mth set of encodeddata slices that includes the encoded data slices 1-M, 2-M, through n-M.

Having produced the sets of encoded data slices 1-M, the DS processing34 and temporarily stores the sets of encoded data slices in the cachememory. As a specific example, the DS processing 34 temporarily stores adecode threshold number of encode data slices for each set of theplurality of sets of encoded data slices. For instance, the DSprocessing 34 temporarily stores, as part of the write operation, thedecode threshold number of encoded data slices for each set of theplurality of sets of encoded data slices for a specified period of time.The specified period of time commences upon initiation of the writeoperation and the write operation is completed prior to expiration ofthe specified period of time. Prior to expiration of the specifiedperiod of time, the DS processing 34 responds to read requests regardingthe data object 222 by retrieving encoded data slices from the cachememory, where the read requests includes at least a read request.

As another specific example of temporarily storing the sets of encodeddata slices in a cache memory, the DS processing 34 temporarily storesthe plurality of sets of encoded data slices. As a particular example,the DS processing 34 stores each set of encoded data slices in a uniquecache memory. For instance, the DS processing stores the first set ofencoded data slices in a cache memory 1, the second set of encoded dataslices in a cache memory 2, through storing the Mth set of encoded dataslices in a cache memory M. As such, a data recovery performanceimprovement may be provided when accessing a single cache memory torecover a corresponding data segment associated with a particular set ofencoded data slices. For instance, the DS processing 34 recovers adecode threshold number of encoded data slices of the first set ofencoded data slices from the cache memory 1 to reproduce a first datasegment.

FIG. 13A illustrates initial steps of an example of the utilizing of thecached encoded data slices where, having temporarily stored encoded dataslices of the plurality of sets of encoded data slices in a cachememory, the DS processing 34 further executes the write operation towrite, via the network 24, the plurality of sets of encoded data slicesto the DSN memory 22. As a specific example, the DS processing 34issues, via the network 24, write commands to storage units of the DSNmemory 22 for writing the plurality of sets of encoded data slices tothe DSN memory 22.

Each write command includes one of an individual write slice request,associated with an individual slice mode, and a batch write slicerequest, associated with a batch slice mode, where the individual writeslice request includes one encoded data slice and the batch writerequest includes two or more encoded data slices. For instance, the DSprocessing 34 issues the batch write slice request to DS unit 1, wherethe batch write slice request includes an encoded data slices 1-1through 1-M (e.g., of a common pillar 1). In another instance, the DSprocessing 34 issues the individual write slice request to DS unit 1,where the individual write slice request includes the encoded data slice1-1; issues another individual write slice request to DS unit 1, wherethe other individual read slice request includes an encoded data slice1-2, etc.

FIG. 13B illustrates further steps of the example of the utilizing ofthe cached encoded data slices where the write commands include thebatch slice mode. Having issued the write commands to the storage unitsof the DSN, the DS processing 34 receives, via the network 24,confirmation from one of the storage units that an identified encodeddata slice of the plurality of sets of encoded data slices has beenstored in the DSN memory 22. In response to receiving the confirmation,the DS processing 34 deletes the identified encoded data slice from thecache memory. For example, the DS processing 34 receives confirmationthat encoded data slices 2-1 through 2-M have been successfully storedin DS unit 2, and deletes the encoded data slices 2-1 through 2-M fromthe cache memory (e.g., deletes encoded data slice 2-1 from cache memory1, deletes encoded data slice 2-2 from cache memory 2, etc.

While executing the write operation, the DS processing 34 receives theread request for at least a portion of the data object 222. Havingreceived the read request, the DS processing 34 identifies relevant setsof encoded data slices of the plurality of sets of encoded data slicesbased on the read request (e.g., which data segments of the M datasegments pertain to the portion). Having identified the relevant sets ofencoded data slices, the DS processing 34 determines whether at leastsome encoded data slices of the relevant sets of encoded data slices arestored in the cache memory. For instance, the DS processing 34 attemptsto retrieve encoded data slices of the relevant sets of encoded dataslices from the cache memory.

When, for a set of encoded data slices of the relevant sets of encodeddata slices, the at least some encoded data slices of the relevant setsof encoded data slices are stored in the cache memory, the DS processing34 determines whether a decode threshold number of encoded data slicesis stored in the cache memory (e.g., determine whether the decodethreshold number have been retrieved from the cache memory). When lessthan the decode threshold number of encoded data slices is stored in thecache memory, the DS processing 34 retrieves, via the network 24,encoded data slices from the DSN memory 22 to obtain the decodethreshold number of encoded data slices. For example, the DS processing34 retrieves the encoded data slices 2-1 through 2-M when the encodeddata slices 2-1 through 2-M are desired to produce the decode thresholdnumber of encoded data slices for each data segment. Having obtained thedecode threshold number of encoded data slices, the DS processing 34decodes the decode threshold number of encoded data slices to recover acorresponding data segment and outputs the recovered corresponding datasegment.

FIG. 13C illustrates further steps of the example of the utilizing ofthe cached encoded data slices where the where the write commandsinclude the individual slice mode. Having issued the write commands tothe storage units of the DSN, the DS processing 34 receives, via thenetwork 24, confirmation from one of the storage units that anidentified encoded data slice of the plurality of sets of encoded dataslices has been stored in the DSN memory 22. In response to receivingthe confirmation, the DS processing 34 deletes the identified encodeddata slice from the cache memory. For example, the DS processing 34receives confirmation that encoded data slices 1-1 through 1-n have beensuccessfully stored in DS units 1-n, and deletes the encoded data slices1-1 through 1-n from the cache memory (e.g., deletes encoded data slices1-1 through 1-n from cache memory 1).

While executing the write operation, the DS processing 34 receives theread request for at least a portion of the data object 222. Havingreceived the read request, the DS processing 34 identifies relevant setsof encoded data slices of the plurality of sets of encoded data slicesbased on the read request (e.g., which data segments of the M datasegments pertain to the portion). Having identified the relevant sets ofencoded data slices, the DS processing 34 determines whether at leastsome encoded data slices of the relevant sets of encoded data slices arestored in the cache memory. For instance, the DS processing 34 attemptsto retrieve encoded data slices of the relevant sets of encoded dataslices from the cache memory.

When, for a set of encoded data slices of the relevant sets of encodeddata slices, the at least some encoded data slices of the relevant setsof encoded data slices are stored in the cache memory, the DS processing34 determines whether a decode threshold number of encoded data slicesis stored in the cache memory (e.g., determine whether the decodethreshold number have been retrieved from the cache memory). When lessthan the decode threshold number of encoded data slices is stored in thecache memory, the DS processing 34 retrieves, via the network 24,encoded data slices from the DSN memory 22 to obtain the decodethreshold number of encoded data slices. For example, the DS processing34 retrieves the encoded data slices 1-1 through 1-n when the encodeddata slices 1-1 through 1-n are desired to produce the decode thresholdnumber of encoded data slices for the first data segment. Havingobtained the decode threshold number of encoded data slices, the DSprocessing 34 decodes the decode threshold number of encoded data slicesto recover a corresponding data segment and outputs the recoveredcorresponding data segment.

FIG. 13D is another flowchart illustrating another example of cachingencoded data slices, which includes many similar steps to FIGS. 6, 11,and 12. The method begins with steps 102, 108, 114, 112, 116, and 118 ofFIG. 6 to produce encoded data slices and to send encoded data slices toa DS unit storage set for storage therein. The method continues withstep 212 of FIG. 12 and then steps 196-198 of FIG. 11 to cache theencoded data slices as temporarily stored encoded data slices. Themethod continues at step 218 where the processing module determineswhether the encoded data slices are available for retrieval from the DSunit storage set based on receiving response from the DS units, whereinthe responses indicates that a corresponding encoded data slice isavailable for retrieval and that a DS unit storage set performance isabove a threshold based on performance indicator. The method advances tosteps 202-204 of FIG. 11 when the processing module determines that theencoded data slices are available for retrieval from the DS unit storageset and that the DS unit storage set performance is above the threshold.

In a retrieval example of operation, the processing module receives aretrieval request and determines whether to retrieve a temporarilystored encoded data slice from the temporary memory or an encoded dataslice from the DS unit in response to the retrieval request. Such adetermination may be based on one or more of a DS unit performanceindicator, system performance indicator, the DS unit reliabilityindicator, and an access latency estimate. Next, the processing modulesends a read request to a DS unit of the DS unit storage set regardingretrieval of the encoded data slice when selecting the encoded dataslice from the DS unit in response to the retrieval request.

FIG. 13E is a flowchart illustrating another example of caching encodeddata slices. In particular, a method is presented for use in conjunctionwith one or more functions and features described in conjunction withFIGS. 1-5, 13A-D, and also FIG. 13E. The method continues at step 230where a processing module of a computing device of a dispersed storagenetwork (DSN) executes a write operation to write a plurality of sets ofencoded data slices to DSN memory, where a data object is dispersedstorage error encoded to produce the plurality of sets of encoded dataslices. The writing includes issuing write commands to storage units ofthe DSN for writing the plurality of sets of encoded data slices to theDSN memory.

The method continues at step 232 where the processing module temporarilystores, as part of the write operation, encoded data slices of theplurality of sets of encoded data slices in cache memory. As a specificexample, the processing module temporarily stores the plurality of setsof encoded data slices in the cache memory (e.g., all slices). Asanother specific example, the processing module temporarily stores adecode threshold number of encode data slices for each set of theplurality of sets of encoded data slices. As yet another specificexample, the processing module temporarily stores the decode thresholdnumber of encoded data slices for each set of the plurality of sets ofencoded data slices for a specified period of time, where the specifiedperiod of time commences upon initiation of the write operation, andwhere the write operation is completed prior to expiration of thespecified period of time. The specified period of time may be associatedwith a time frame where a retrieval performance level associated withretrieving the data from the cache memory is desired.

The method continues at step 234 where the processing module receivesconfirmation from one of the storage units that an identified encodeddata slice of the plurality of sets of encoded data slices has beenstored in the DSN memory. In response to receiving the confirmation, themethod continues at step 236 where the processing module deletes theidentified encoded data slice from the cache memory.

While executing the write operation, the method continues at step 238where the processing module receives a read request for at least aportion of the data object. The method continues at step 240 where theprocessing module identifies relevant sets of encoded data slices of theplurality of sets of encoded data slices based on the read request. Forexample, the processing module identifies sets of encoded data slicesassociated with the portion of the data object based on a receivedoffset value.

The method continues at step 242 where the processing module determineswhether at least some encoded data slices of the relevant sets ofencoded data slices are stored in the cache memory. For example, theprocessing module attempts to retrieve encoded data slices from thecache memory. When temporarily storing the decode threshold number ofencoded data slices for each set of the plurality of sets of encodeddata slices for a specified period of time, prior to expiration of thespecified period of time, the processing module response to readrequests regarding the data object by retrieving encoded data slicesfrom the cache memory, where the read requests include the read request.

When, for a set of encoded data slices of the relevant sets of encodeddata slices, the at least some encoded data slices of the relevant setsof encoded data slices are stored in the cache memory, the methodcontinues at step 244 where the processing module determines whether adecode threshold number of encoded data slices is stored in the cachememory. When less than the decode threshold number of encoded dataslices is stored in the cache memory, the method continues at step 246where the processing module retrieves encoded data slices from the DSNmemory to obtain the decode threshold number of encoded data slices. Forexample, the processing module determines a number of encoded dataslices to retrieve from the DSN as a difference between the decodethreshold number and a number of encoded data slices retrieved from thecache memory.

The method continues at step 248 where the processing module decodes thedecode threshold number of encoded data slices to recover acorresponding data segment. The method continues at step 250 where theprocessing module outputs the recovered corresponding data segment.

The method described above in conjunction with a processing module canalternatively be performed by other modules of a dispersed storagenetwork or by other devices. In addition, at least one memory sectionthat stores operational instructions can, when executed by one or moreprocessing modules of one or more computing devices of a dispersedstorage network (DSN), cause the one or more computing devices toperform any or all of the method steps described above.

As may be used herein, the terms “substantially” and “approximately”provides an industry-accepted tolerance for its corresponding termand/or relativity between items. Such an industry-accepted toleranceranges from less than one percent to fifty percent and corresponds to,but is not limited to, component values, integrated circuit processvariations, temperature variations, rise and fall times, and/or thermalnoise. Such relativity between items ranges from a difference of a fewpercent to magnitude differences. As may also be used herein, theterm(s) “operably coupled to”, “coupled to”, and/or “coupling” includesdirect coupling between items and/or indirect coupling between items viaan intervening item (e.g., an item includes, but is not limited to, acomponent, an element, a circuit, and/or a module) where, for indirectcoupling, the intervening item does not modify the information of asignal but may adjust its current level, voltage level, and/or powerlevel. As may further be used herein, inferred coupling (i.e., where oneelement is coupled to another element by inference) includes direct andindirect coupling between two items in the same manner as “coupled to”.As may even further be used herein, the term “operable to” or “operablycoupled to” indicates that an item includes one or more of powerconnections, input(s), output(s), etc., to perform, when activated, oneor more its corresponding functions and may further include inferredcoupling to one or more other items. As may still further be usedherein, the term “associated with”, includes direct and/or indirectcoupling of separate items and/or one item being embedded within anotheritem. As may be used herein, the term “compares favorably”, indicatesthat a comparison between two or more items, signals, etc., provides adesired relationship. For example, when the desired relationship is thatsignal 1 has a greater magnitude than signal 2, a favorable comparisonmay be achieved when the magnitude of signal 1 is greater than that ofsignal 2 or when the magnitude of signal 2 is less than that of signal1.

While the transistors in the above described figure(s) is/are shown asfield effect transistors (FETs), as one of ordinary skill in the artwill appreciate, the transistors may be implemented using any type oftransistor structure including, but not limited to, bipolar, metal oxidesemiconductor field effect transistors (MOSFET), N-well transistors,P-well transistors, enhancement mode, depletion mode, and zero voltagethreshold (VT) transistors.

The present invention has also been described above with the aid ofmethod steps illustrating the performance of specified functions andrelationships thereof. The boundaries and sequence of these functionalbuilding blocks and method steps have been arbitrarily defined hereinfor convenience of description. Alternate boundaries and sequences canbe defined so long as the specified functions and relationships areappropriately performed. Any such alternate boundaries or sequences arethus within the scope and spirit of the claimed invention.

The present invention has been described, at least in part, in terms ofone or more embodiments. An embodiment of the present invention is usedherein to illustrate the present invention, an aspect thereof, a featurethereof, a concept thereof, and/or an example thereof. A physicalembodiment of an apparatus, an article of manufacture, a machine, and/orof a process that embodies the present invention may include one or moreof the aspects, features, concepts, examples, etc. described withreference to one or more of the embodiments discussed herein.

The present invention has been described above with the aid offunctional building blocks illustrating the performance of certainsignificant functions. The boundaries of these functional buildingblocks have been arbitrarily defined for convenience of description.Alternate boundaries could be defined as long as the certain significantfunctions are appropriately performed. Similarly, flow diagram blocksmay also have been arbitrarily defined herein to illustrate certainsignificant functionality. To the extent used, the flow diagram blockboundaries and sequence could have been defined otherwise and stillperform the certain significant functionality. Such alternatedefinitions of both functional building blocks and flow diagram blocksand sequences are thus within the scope and spirit of the claimedinvention. One of average skill in the art will also recognize that thefunctional building blocks, and other illustrative blocks, modules andcomponents herein, can be implemented as illustrated or by discretecomponents, application specific integrated circuits, processorsexecuting appropriate software and the like or any combination thereof.

What is claimed is:
 1. A method for execution by a computing device of adispersed storage network (DSN), the method comprises: executing a writeoperation to write a plurality of sets of encoded data slices to DSNmemory, wherein a data object is dispersed storage error encoded toproduce the plurality of sets of encoded data slices and wherein thewrite operation includes temporarily storing encoded data slices of theplurality of sets of encoded data slices in cache memory; and whileexecuting the write operation: receiving a read request for at least aportion of the data object; identifying relevant sets of encoded dataslices of the plurality of sets of encoded data slices based on the readrequest; determining whether at least some encoded data slices of therelevant sets of encoded data slices are stored in the cache memory;when, for a set of encoded data slices of the relevant sets of encodeddata slices, the at least some encoded data slices of the relevant setsof encoded data slices are stored in the cache memory: determiningwhether a decode threshold number of encoded data slices is stored inthe cache memory; when less than the decode threshold number of encodeddata slices is stored in the cache memory, retrieving encoded dataslices from the DSN memory to obtain the decode threshold number ofencoded data slices; decoding the decode threshold number of encodeddata slices to recover a corresponding data segment; and outputting therecovered corresponding data segment.
 2. The method of claim 1 furthercomprises: temporarily storing the decode threshold number of encodedata slices for each set of the plurality of sets of encoded dataslices.
 3. The method of claim 1 further comprises: temporarily storingthe plurality of sets of encoded data slices.
 4. The method of claim 1further comprises: issuing write commands to storage units of the DSNfor writing the plurality of sets of encoded data slices to the DSNmemory; receiving confirmation from one of the storage units that anidentified encoded data slice of the plurality of sets of encoded dataslices has been stored in the DSN memory; and in response to receivingthe confirmation, deleting the identified encoded data slice from thecache memory.
 5. The method of claim 1 further comprises: the writeoperation includes temporarily storing the decode threshold number ofencoded data slices for each set of the plurality of sets of encodeddata slices for a specified period of time, wherein the specified periodof time commences upon initiation of the write operation, and whereinthe write operation is completed prior to expiration of the specifiedperiod of time; and prior to expiration of the specified period of time,responding to read requests regarding the data object by retrievingencoded data slices from the cache memory, wherein the read requestsinclude the read request.
 6. A computer readable storage mediumcomprises: at least one memory section that stores operationalinstructions that, when executed by one or more processing modules ofone or more computing devices of a dispersed storage network (DSN),causes the one or more computing devices to: execute a write operationto write a plurality of sets of encoded data slices to DSN memory,wherein a data object is dispersed storage error encoded to produce theplurality of sets of encoded data slices and wherein the write operationincludes temporarily storing encoded data slices of the plurality ofsets of encoded data slices in a cache memory; and while executing thewrite operation: receive a read request for at least a portion of thedata object; identify relevant sets of encoded data slices of theplurality of sets of encoded data slices based on the read request;determine whether at least some encoded data slices of the relevant setsof encoded data slices are stored in the cache memory; when, for a setof encoded data slices of the relevant sets of encoded data slices, theat least some encoded data slices of the relevant sets of encoded dataslices are stored in the cache memory: determine whether a decodethreshold number of encoded data slices is stored in the cache memory;when less than the decode threshold number of encoded data slices isstored in the cache memory, retrieve encoded data slices from the DSNmemory to obtain the decode threshold number of encoded data slices;decode the decode threshold number of encoded data slices to recover acorresponding data segment; and output the recovered corresponding datasegment.
 7. The computer readable storage medium of claim 6 furthercomprises: the at least one memory section stores further operationalinstructions that, when executed by the one or more processing modules,causes the one or more computing devices of the DSN to: temporarilystore the decode threshold number of encode data slices for each set ofthe plurality of sets of encoded data slices.
 8. The computer readablestorage medium of claim 6 further comprises: the at least one memorysection stores further operational instructions that, when executed bythe one or more processing modules, causes the one or more computingdevices of the DSN to: temporarily store the plurality of sets ofencoded data slices.
 9. The computer readable storage medium of claim 6further comprises: the at least one memory section stores furtheroperational instructions that, when executed by the one or moreprocessing modules, causes the one or more computing devices of the DSNto: issue write commands to storage units of the DSN for writing theplurality of sets of encoded data slices to the DSN memory; receiveconfirmation from one of the storage units that an identified encodeddata slice of the plurality of sets of encoded data slices has beenstored in the DSN memory; and in response to receiving the confirmation,delete the identified encoded data slice from the cache memory.
 10. Thecomputer readable storage medium of claim 6 further comprises: the atleast one memory section stores further operational instructions that,when executed by the one or more processing modules, causes the one ormore computing devices of the DSN to: temporarily store, as part of thewrite operation, the decode threshold number of encoded data slices foreach set of the plurality of sets of encoded data slices for a specifiedperiod of time, wherein the specified period of time commences uponinitiation of the write operation, and wherein the write operation iscompleted prior to expiration of the specified period of time; and priorto expiration of the specified period of time, respond to read requestsregarding the data object by retrieving encoded data slices from thecache memory, wherein the read requests include the read request.
 11. Acomputing device of a dispersed storage network (DSN), the computingdevice comprises: an interface; a cache memory; and a processing moduleoperably coupled to the interface and the cache memory, wherein theprocessing module functions to: execute a write operation to write, viathe interface, a plurality of sets of encoded data slices to DSN memory,wherein a data object is dispersed storage error encoded to produce theplurality of sets of encoded data slices and wherein the write operationincludes temporarily storing encoded data slices of the plurality ofsets of encoded data slices in the cache memory; and while executing thewrite operation: receive a read request for at least a portion of thedata object; identify relevant sets of encoded data slices of theplurality of sets of encoded data slices based on the read request;determine whether at least some encoded data slices of the relevant setsof encoded data slices are stored in the cache memory; when, for a setof encoded data slices of the relevant sets of encoded data slices, theat least some encoded data slices of the relevant sets of encoded dataslices are stored in the cache memory: determine whether a decodethreshold number of encoded data slices is stored in the cache memory;when less than the decode threshold number of encoded data slices isstored in the cache memory, retrieve, via the interface, encoded dataslices from the DSN memory to obtain the decode threshold number ofencoded data slices; decode the decode threshold number of encoded dataslices to recover a corresponding data segment; and output the recoveredcorresponding data segment.
 12. The computing device of claim 11,wherein the processing module further functions to: temporarily storethe decode threshold number of encode data slices for each set of theplurality of sets of encoded data slices.
 13. The computing device ofclaim 11, wherein the processing module further functions to:temporarily store the plurality of sets of encoded data slices.
 14. Thecomputing device of claim 11, wherein the processing module furtherfunctions to: issue, via the interface, write commands to storage unitsof the DSN for writing the plurality of sets of encoded data slices tothe DSN memory; receive, via the interface, confirmation from one of thestorage units that an identified encoded data slice of the plurality ofsets of encoded data slices has been stored in the DSN memory; and inresponse to receiving the confirmation, delete the identified encodeddata slice from the cache memory.
 15. The computing device of claim 11,wherein the processing module further functions to: temporarily store,as part of the write operation, the decode threshold number of encodeddata slices for each set of the plurality of sets of encoded data slicesfor a specified period of time, wherein the specified period of timecommences upon initiation of the write operation, and wherein the writeoperation is completed prior to expiration of the specified period oftime; and prior to expiration of the specified period of time, respondto read requests regarding the data object by retrieving encoded dataslices from the cache memory, wherein the read requests include the readrequest.